Localization of chromosomal protein HMG-1 in polytene chromosomes of Chironomus thummi

Clusters of nucleosomes containing chromosomal protein HMG-17 in chromatin

Chromosomal proteins HMG-14 and HMG-17 are nucleosome binding proteins which can function as architectural elements to alter the structure of the chromatin fiber and enhance transcription from chromatin templates. Here we study the spatial organization of these HMG proteins in the nucleus and the distribution of nucleosomes containing HMG-17 in the chromatin fiber. By confocal immunofluorescence microscopy we find that HMG-14/17 proteins are clustered into foci containing either HMG-14 or HMG-17. These results suggest that HMG-14/17 proteins segregate into distinct nuclear domains. Indeed, immunofractionation of defined length oligonucleosomes, with affinity pure antibodies to HMG-17, indicates that oligonucleosomes containing HMG-17 are devoid of HMG-14. Quantitative analysis indicates that in cellular chromatin nucleosomes containing HMG-17 are clustered. The average size of the cluster is six contiguous HMG-17-containing nucleosomes. The nucleosomes in this cluster contain either two or zero molecules of HMG-17 and a complete set of four core histones. We suggest that HMG-14/17 proteins modify the nucleosomal organization of the 30 nm chromatin fiber, to unfold the higher order chromatin structure and facilitate access to the underlying DNA sequence. Clustering of architectural elements, such as HMG proteins and linker histone subtypes into distinct domains, may lead to structural and functional heterogeneity along the chromatin fiber.

Immunochemical approaches to the study of histone H1 and high mobility group chromatin proteins

This review is an attempt to summarize all existing data on histone H1 and high mobility group proteins obtained with immunochemical methods. The following issues are treated consecutively: production of specific antisera to these protein groups, antigenic structure of the polypeptide chains, use of antibodies for the identification, the quantitative estimation and the study of the tissue- and species-specificity of the proteins. Special attention is devoted to the studies of the localization of the respective antigens in the cell, the nucleus, the chromosomes and the interphase chromatin. The use of specific antibodies for the elucidation of the role these proteins play in such basic cellular processes as proliferation and differentiation, replication and transcription is also discussed. It becomes clear that the use of immunochemical approaches in the study of specific chromatin proteins both at the level of the protein molecule and at the level of chromatin can be a powerful tool for the resolution of a number of specific problems. The field is very promising and will undoubtedly develop intensely in the nearest future.

Histone H1 in the centromeric heterochromatin of Glyptotendipes barbipes larval polytene chromosomes

Immunofluorescent analysis with antibodies against histone H1 failed to detect H1 in the centromeric heterochromatin blocks of the polytene chromosomes of Glyptotendipes barbipes larvae. Centromeric regions were dissected microsurgically and acid-extracted. Electrophoresis in SDS and acid-urea gels revealed a band comigrating with H1 of calf thymus and of Gl. barbipes salivary gland nuclei. ELISA dot assay of the extracted material gave a positive reaction with anti-H1 monoclonal antibodies and with anti-H1 affinity-purified polyclonal antibodies. This shows that the centromeric heterochromatin contains histone H1 but packed in a way which prevents the H1 antigenic determinants from reacting in situ with the specific antibodies.

Microscopic imaging of cells

The microworld was revealed to investigators through a glass bead or a hanging water droplet long before optics was understood. The cellular structure of plants was well resolved by such simple magnifying glasses, van Leeuwenhoek, the Dutch merchant and amateur microscopist, was the first to report to the English Royal Society his observations of bacteria with his single-lens microscope in 1665.

Localization of nuclear proteins related to high mobility group protein 14 (HMG 14) in polytene chromosomes

An antibody was raised against "high mobility group" nuclear protein 14 (HMG 14) from calf thymus, known to be associated with actively transcribed chromatin. By means of indirect immunofluorescence, it was shown to react with the nuclei of mouse fibroblasts and of brain cells from Xenopus and Drosophila, but not of Xenopus erythrocytes. The antibody was used to detect immunologically related proteins in giant chromosomes of the midge, Chironomus pallidivittatus. Indirect immunofluorescence with anti-HMG 14 antibody in polytene nuclei was restricted to the active puffs. Giant puffs (Balbiani rings) exhibited especially intense fluorescence in their peripheral regions. An inducible puff site, the Balbiani ring 6 locus, showed no reaction with the antibody prior to induction. When puff formation began, the chromosome site assumed a very intense fluorescence, which disappeared again when the Balbiani ring was recondensed. - Protein extracts of salivary gland nuclei were found on immunoblots to contain one major protein fraction that reacted with the anti-HMG 14 antibody. The electrophoretic mobility of this fraction was similar to that of calf thymus HMG 17. - It is concluded that actively transcribed puffs in polytene chromosomes contain HMG 14-related protein(s) that are not present in potentially active gene loci prior to induction.

Transcriptionally active chromatin

Eukaryotic chromatin has a dynamic, complex hierarchical structure. Active gene transcription takes place on only a small proportion of it at a time. While many workers have tried to characterize active chromatin, we are still far from understanding all the biochemical, morphological and compositional features that distinguish it from inactive nuclear material. Active genes are apparently packaged in an altered nucleosome structure and are associated with domains of chromatin that are less condensed or more open than inactive domains. Active genes are more sensitive to nuclease digestions and probably contain specific nonhistone proteins which may establish and/or maintain the active state. Variant or modified histones as well as altered configurations or modifications of the DNA itself may likewise be involved. Practically nothing is known about the mechanisms that control these nuclear characteristics. However, controlled accessibility to regions of chromatin and specific sequences of DNA may be one of the primary regulatory mechanisms by which higher cells establish potentially active chromatin domains. Another control mechanism may be compartmentalization of active chromatin to certain regions within the nucleus, perhaps to the nuclear matrix. Topological constraints and DNA supercoiling may influence the active regions of chromatin and be involved in eukaryotic genomic functions. Further, the chromatin structure of various DNA regulatory sequences, such as promoters, terminators and enhancers, appears to partially regulate transcriptional activity.

Selective exposure of antigenic determinants in chromosomal proteins upon gene activation in polytene chromosomes

The immunological accessibility of the nucleosomal core protein H3 and non-histone protein, HMG-1, was studied in transcriptionally active regions of Chironomus thummi polytene chromosomes. Chromosomal loci were decondensed by incubating isolated salivary glands in various salt solutions or hemolymph. Indirect immunofluorescence studies on these chromosomes using anti-sera to histone H3 revealed that the puffed regions were depleted of fluorescence. The lack of fluorescence could be correlated with the degree of puffing and the level of transcriptional activity. The puffed regions fluoresce after anti-H3 addition if the chromosomes are not cross-linked with formaldehyde, and if prior to the addition of antibodies the chromosomes are treated with 45% acetic acid. We conclude that, whereas histone H3 is present in the puffed regions, its antigenic determinants are sterically hindered by components which are extractable by 45% acetic acid. On the other hand, the antigenic determinants of protein HMG-1 are always available to antibody binding in puffed regions, as well as other chromosomal areas.

Immunoelectron microscopic localization of RNA polymerase B on isolated polytene chromosomes of Chironomus tentans

RNA polymerase B (or II) was localized by immunoelectron microscopy in ultrathin sections of polytene chromosomes isolated from larval salivary glands of Chironomus tentans. The enzyme was found at decondensed sites (puffs and interbands), whereas no detectable RNA polymerase B was present in condensed loci (bands). Within each of the large puffs the highest enzyme concentration was observed wherever the chromatin was in the most decondensed state. Otherwise the enzyme appeared homogeneously distributed within puffs and interbands. This immunoelectron microscopic study, along with the recently published immunofluorescent and autoradiographic analysis of isolated Chironomus chromosomes (Sass, 1982) unequivocally demonstrates that RNA polymerase B is present in most, if not all interbands.